One conventional method of supplying duplex signaling for telephone trunk networks is performed using a four-winding polar relay in the signaling circuit of the originating office and a like polar relay in the terminating office. Each relay is operated in response to current flow in the proper combination of windings, the current being of the proper direction and magnitude. The polar relay in the originating office responds to originating office conditions to provide a signal on the transmission or M lead and applies signals to the receive or E lead in the terminating office accordingly. The relay in the originating office responds to the answer condition in the terminating office to provide supervisory control.
In replacing the four-winding relay with solid state components, at least one known approach is that of using a circuit employing two differential amplifiers in place of each four-winding relay. The amplifiers sense the input conditions to sum the results in reaching an operating threshold to a further pair of amplifiers. A plurality of settable straps in combination with potentiometers allow the circuit to be adjusted for line variables.